This invention pertains to a turntable mechanism. In a more particular aspect the invention relates to a turntable mechanism intended for use in manipulating a part to be sprayed by a jet of plasma within a vacuum plasma spray chamber. In still another of its embodiments a turntable is provided which is capable of supporting heavy parts such as aircraft engines.
The internal parts and surfaces of turbine or rocket engines and similar parts, such as combustion chambers and turbine blades are subjected to such high temperatures during operation that it is desirable to coat their surfaces with thermal-barrier ceramic coatings. In order to do that the part must be both rotated and tilted. A turntable must therefore be provided which not only makes it possible to rotate and tilt a heavy workpiece, but to operate at vacuum plasma temperatures to do so. Such devices as that shown in U.S. Pat. No. 3,046,157 would not support a turbine engine. Devices such as those described in U.S. Pat. No. 2,783,531 and U.S. Pat. No. 2,835,964, with motors close to the turntable would not withstand high temperatures without burning out the motors.
Vacuum plasma coating with ceramics and high temperature alloys such as zirconium and yttrium entails spraying a jet of ceramic or alloy plasma, in the absence of air, at a temperature of approximately 15,000 degrees C. A plasma spray gun begins to operate when a pulse of current creates an arc across the gap between its electrodes. An inert gas, usually argon in admixture with hydrogen, flows within the arc. As the arc forms, electrons are stripped away from the gas, ionizing it so that it forms a plasma. When a powdered ceramic material or alloy is introduced into this plasma stream the particles are melted by the high temperatures and propelled as a plasma onto a workpiece surface where they solidify, forming a high temperature resistant coating virtually impossible to remove.
In coating such parts as combustion chambers of turbine or rocket engines the workpiece must not only be rotated, but it must be tilted so that the surface being sprayed is approximately perpendicular to the spray gun. Thus the turntable must not only be capable of supporting 750 to 1000 pounds, but of angulating such a heavy workpiece by rotating it about both vertical and horizontal axes. And this must be done without drive means failure due to the extremely high temperatures under which the drive means must operate. Obviously the turntable mechanisms of U.S. Pat. No. 2,783,531 and U.S. Pat. No. 2,835,964 could not be so used. Herein a turntable mechanism is provided which can be so used.